Fence systems that are capable of sensing tamper related activity on the fence wire are well known in the containment and security industries. A taut wire security fence system is one in which one or more wires are held in tension between two end supports. An end support may be any support where the wire of the fence is terminated, such as at a fixed post or another fixed physical structure (for example, a wall). A taut wire fence may include a number of intermediate supports, but need not do so. Reference to a taut wire fence throughout this specification should be understood to include an electrified taut wire fence as well as a conventional, non-electric, taut wire fence, unless specifically stated otherwise.
One means of breaching an electric security fence is to cut the wires of the fence. This kind of breach is easily detected by electrical signals that are interrupted by a cut wire and can then sound an alarm for the particular security zone being breached.
Another method an intruder can breach an electric taut wire security fence is by climbing the fence. This can result in the fence wires deflecting downwards and connecting together as a short circuit which can be detected as loss or reduction of electrical signal through a wire.
Yet another method of breaching an electric fence is to deflect the wires of the fence, as can occur by a person climbing onto the wire or otherwise pulling the wires apart to enable an intruder or object to pass through the fence. In such instances the wires may not short or open circuit the signal through the fence. However, in both electric and conventional fences (not electrified) cutting, bending or spreading the wires changes the tension in the wires, which can be detected by a sensor.
However, in many instances a taut wire fence is not an electric fence and hence tamper activities cannot be detected through an electrical disturbance as with an electric fence. In general a system to detect an attempt to breach or tamper with a taut wire security fence would ideally work for both electric and conventional (non-electric) taut wire fences.
U.S. Pat. No. 6,646,563 (to Buckley et al) discloses a deflection system for a taut wire fence including a plate member configured such that a wire of a taut wire fence can be wound around the plate member between two attachment points, one on each side of the plate member. The plate member is pivotally mounted on an intermediate fence support (e.g., a post) such that a change of tension of the wire on either side of the plate member causes the plate member to rotate about the pivot mount. A sensor mounted on the plate member between the attachment points is used to detect a change in strain exerted on the plate as it is rotated by the change in tension in the wire. The sensor is connected to a sensor processing circuit for interrogating the sensor and raising an alarm if tampering is detected.
However, this system has a number of disadvantages, including the requirement for the device to be mounted onto a support post. Not all taut wire fence systems include intermediate support posts. Further, the location of the device on a post can enable the system to be circumvented readily by an intruder using the post as an anchor to clamp the plate member (or a wire) thus preventing the plate member from pivoting (or preventing a change in tension from being transferred to the plate member).
A further disadvantage is that the sensitivity of the sensor to a change in tension can change (decrease quite rapidly) as the plate member pivots. Thus, while the sensor may be initially more or less in line with the wires it is connected to (in fact it is never quite in line as the wires on either side of the plate member are not in line due to their being twisted around from one side of the plate member to the other), as the plate member pivots the sensor is subjected to a much smaller change in tension than would be the case if the sensor was truly in line with the wires on either side of it and stayed that way as one of the wires is deflected.
A further disadvantage with the system disclosed by Buckley et al. is that it is configured to detect a change in deflection of one or other of the wires, causing the plate member to pivot, rather than the tension itself. If, for example one (or both) of the wires on either side of the plate member is cut, the plate member would not pivot.
The device of Buckley can also create problems if for any reason the fence requires re-tensioning, as may be required from time to time for maintenance of the taut wire fence. If the plate member is retained on its pivot mount to the support post and connected to the wire, then tensioning the wire on one side will have no effect on the tension on the wire on the other side of the plate member as the pivot connection of the plate to the post prevents the plate from moving and thus transferring the tension to the wire on the other side. Re-tensioning the wires on both sides of the plate member could entail tensioning both wires to the same tension independently, or disconnection of the plate member from the wires, re-tensioning the wire and reconnecting the plate member to the wire. Either way, this process can take more time than is normally required to re-tension a fence wire.
Security systems that are in-line with the wires and are not post mounted have been disclosed in U.S. Pat. No. 5,371,488 (to Couch et al) and U.S. Pat. No. 6,891,472 (to Tallman). The devices disclosed by Couch et al. and Tallman both involve connection of two separate wires which are constrained within the device but can move relative to one another as a result of a change in tension of one or other of the wires. Both devices include sensors which are located in fixed positions relative to the housing of the device. The sensors are configured to detect when one or other of the wires moves beyond a predetermined point(s) within the housing. In other words, both devices are essentially trip devices, physically configured to detect when a wire moves within the housing beyond a predetermined amount (typically as a result of tampering with the wire) and trigger an alarm when this occurs.
A disadvantage with this type of device is that they can be readily circumvented. As both devices rely on movement of the wires relative to the housing of the device, an intruder can readily disable the devices by clamping the wires outside the device to the housing of the device, or equivalently, clamping the wires on either side of the device together so that they cannot move relative to one another. It is also not clear how the devices would perform if one or other of the connected wires were cut.
Further, the devices are not configured to measure the actual tension in the wires or how this may change over time, and therefore cannot provide information relating to the condition of the fence at all times. In particular, they are generally unable to detect the different characteristics of an attempt to tamper with the fence and a change in tension due to other events, such as an animal pushing against the wires. As both can lead to changes in tension, these devices are prone to providing false alarms (i.e. alarms raised due to events other than human tampering),
One common problem with each of these devices is that the sensor or tensile force gauge does not measure the tension in the wire directly, but rather the balance of the tensions on either side of the micro switch or tensile force gauge. The sensor/gauge will therefore normally be in a neutral or zero condition. As a result, this arrangement cannot detect a change in tension by the same amount on both sides of the switch or gauge. Such changes can occur over time due to stretching of the wire or temperature variations for example, in each case reducing the effectiveness of the taut wire fence and the detection devices.
It is generally important for the tension along the wire of a taut wire fence system to be kept within a prescribed range of tensions for effective operation of the fence under the conditions and purpose for which it is to be used. A sensor/gauge system of the prior art devices discussed above cannot detect an overall change in the tension, for whatever reason, which can result in the tension dropping below the desired range without activation of an alarm.
If the wire of the fence becomes sufficiently loose over time the sensor/gauge may not detect further deflection due to tampering.
A further problem with these systems is that they can be prone to false alarms. Typically, the alarm is triggered by the device detecting a change of tension, or a change in excess of a predetermined threshold. Such systems cannot differentiate between a tension change due to a deliberate attempt to tamper, and, for example, an accidental change due to an animal pushing against the wire.
Another problem with these arrangements in general is that the wire(s) cannot be re-tensioned without disconnecting the detector from the wire. This can add time and cost to maintenance and is generally inconvenient.
Another problem is the high cost of installation of the sensors and the large number of them required when they are attached to each support post and each wire of a security fence system.
It is an object of the present invention to address the foregoing problems or at least to provide the public with a useful choice.
One object of this invention is to provide a system that can sense the tampering of security electric fence wires. Another object is to provide a system that may be easy to install and which may improve the rejection rate of false alarms and may cope with changes of tension in the wires due to temperature changes and other environmental factors which are not tamper conditions.
The preferred application of the invention is for use with electrified security fences but this should not be seen as limiting as it may be used in non electrified security fence applications as well.
It is acknowledged that the term ‘comprise’ may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term ‘comprise’ shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term ‘comprised’ or ‘comprising’ is used in relation to one or more steps in a method or process.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.